Mark,
Not a lot to add. I would say most back bunkers are for visual framing, as statistically, they rarely come into play barring some unusual factors. They are built above the green for visibility, but if their front lips aren't within a few feet of the green, they turn out to be mostly cosmetic.
One reason to build the up from the green, is that you can put a small bunker, grass bunker or swale right behind the green as hazards, but it will be blind, and then you build the bunker up higher to "mark" the hazard, even if you aren't seeing the one that really affects your shot.
The 4 bunkers behind ANGC were rebuilt years ago with a swale in front, because formerly, water came down the hill, was directed between the bunkers, and on to the putting surface. Someone posted a picture of the JN/Bob Cupp grading plan for that years ago. I have it, but currently can't seem to post photos on gca.com.